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The right engine of J-1 accelerates and ejects high temperature and high pressure gas backward. Because the forces are mutual, the gas gives the fighter a forward reaction, and the fighter can accelerate forward and fly out of the deck.

When the fighter plane flies off the deck, the total gravity of the aircraft carrier becomes smaller. Because it is still floating, buoyancy is equal to gravity, and buoyancy becomes smaller.

Therefore, the answer is: the role of force is mutual; decrease